Conservative MPs overwhelmingly backed Sunak to become Britain's youngest prime minister in modern times.
The path to office for the former chancellor was cleared on Sunday when Boris Johnson stopped trying to get back into politics. Johnson claimed he could have won the contest despite securing the support of over 50 MPs.
The deadline for nominations for the Conservative party leadership was 2pm on Monday, but Sunak's last remaining rival withdrew minutes before then.
The leader of the House of Commons got the nominations of 90 Tories, less than the 100 needed to enter the contest. She urged the party to unite behind Sunak.
The contest ended without an online vote because Sunak was the only candidate.
Tony Blair was younger than Sunak when he was prime minister. His parents came to Britain from east Africa and he was born there.
Sunak is a divisive figure in the Conservative party, with many of his supporters accusing him of damaging Johnson's reputation when he quit as chancellor.
Sunak now faces a crisis in the public finances, high inflation, stretched public services, industrial unrest, soaring energy bills, and the possibility of black outs in the New Year.
Sunak promised integrity, professionalism and accountability at every level of the government he leads.
Financial markets responded positively to Sunak's expected "coronation" on Monday in what has been dubbed in the City of London "the dullness dividend".
Sunak put up taxes in order to restore order to the finances that were stretched by the Covid-19 epidemic.
Jeremy Hunt is expected to remain as Sunak's chancellor. The Treasury believes that a medium-term debt-cutting plan will be presented to parliament on October 31st.
The task of assembling a new cabinet is the first thing Sunak has to do to unite the Conservatives.
King Charles could invite Sunak to form a government as early as Monday if the transfer of power goes according to plan.
As investors bet that Britain would stick with policies outlined by Hunt that have calmed markets in recent days, gilts and sterling rose.
The yield on the 10-year gilt fell by 0.24 percentage points to 3.81 per cent early on Monday. The pound increased in value against the dollar.